Magnetic tape cassette, cassette half and molding method therefor

ABSTRACT

The cassette half molding method molds one of upper and lower halves of the magnetic tape cassette by injection molding with an injection mold in which a gas vent pin for smoothing a flow of molten resin to be introduced in the cavity for molding a transparent or semitransparent window portion provided in the upper half. The cassette half is molded by this molding method. The magnetic tape cassette has a friction sheet interposed either between a pair of take-up hubs and the upper half, or between the pair of take-up hubs and the lower half, or both, and the friction sheet is made of polyethylene-based resin and has a surface electric resistance value in the range of 1×10 7  Ω to 1×10 10  Ω. The magnetic tape cassette has a rib for reducing a resistance when the roller of the detecting member passes through which is provided at a position corresponding to a recess and on a side of the front lid in the top surface of the body case.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a technical field of a magnetictape cassette and an injection molding method in case of molding a resinproduct for the magnetic tape cassette with molding dies, and moreparticularly, relates to a cassette half for forming a case of themagnetic tape cassette for accommodating a magnetic tape wound around apair of take-up hubs and a molding method for molding the cassette half,a magnetic tape cassette provided with an improved friction sheet forpreventing static charge of the magnetic tape and keeping slidability ofthe magnetic tape, and a magnetic tape cassette improved indetachability to a cassette automatic delivery apparatus.

[0003] 2. Description of the Related Art

[0004] For example, a magnetic tape cassette used to an audio system ora computer may be exemplified as one example of the resin product. Ingeneral, the magnetic tape cassette has a structure in which a pair ofhubs around which the magnetic tape is wound are rotatably incorporatedand retained in a case body formed of upper and lower cassette halvescoupled together in a lid-alignment manner. Such cassette halves areinjection molded of thermoplastic resin such as PS (polystyrene), or ABS(acrylonitrile butadiene styrene resin) general. In order to observe andconfirm the winding condition of the magnetic tape from the outside, thecassette halves as a whole are made of transparent material such as AS(acrylonitrile styrene) or PS, or a window portion that is transparentor semitransparent is formed in the opaque cassette half bodies.

[0005] Conventionally, this window portion is formed by adhesion withadhesive or supersonic welding of the window material molded separatelyfrom the cassette half bodies. However, in this case, since the moldingstep and adhesion step for the window portion are required, there is arestriction on a design and productivity is not so good. Accordingly,recently, the cassette half bodies and window portions are integrallymolded through a color injection molding (or multi-color injectionmolding) method.

[0006]FIG. 8 is a plan view of an upper half of a conventional magnetictape cassette.

[0007] A window portion 112 that is transparent or semitransparent isprovided in the upper half 110 for observing and confirming the windingcondition of the magnetic tape accommodated in the cassette halves fromthe outside. A pair of ribs 114 and 116 are provided for positioning andregulating the take-up hubs around which the magnetic tape is wound inthis window portion 112. These ribs 114 and 116 are divided into twoportions, for example, 114 a and 114 b, and arranged in thecircumferential direction in conformity with the shape of the take-uphubs.

[0008] A cross-sectional view taken along the line IX-IX of FIG. 8 isshown in FIG. 9.

[0009] As shown in FIG. 9, position regulating ribs 114 (114 a, 114 b)of the take-up hubs are formed in the window portion 112 of the upperhalf 110 with a height in such an appropriate range that they canregulate the position of the take-up hubs and do not prevent therotation of the take-up hubs.

[0010] A state that the window portion 112 is molded by means of, forexample, the two-color injection molding method is shown in FIG. 10.

[0011] Molten resin is injected into a space (cavity) surrounded by afront mold (cavity side part) 120 a and a movable half (force side part)120 b of the molding dies and a slide core 122 to form the upper halfbody 110. Thereafter, as shown in FIG. 10, the slide core 122 is slid bya thickness of the window portion 112. Transparent or semitransparentresin is injected from a gate 126 into the space (cavity) 124 formedtherein to form the window portion 112.

[0012] At this time, recesses 128 (128 a, 128 b) for take-up hubposition regulating ribs 114 (114 a, 114 b) to be formed in the windowportion 112 are formed in the slide core 122.

[0013] However, in the case where the window portion 112 of the upperhalf of the magnetic tape cassette is formed by the injection molding asshown in FIGS. 8 and 9 like the above-described conventional case, thereare the following defects. Namely, as shown in FIG. 10, the molten resininjected into the cavity 124 from the gate 126 flows as indicated by thearrows. In the flat portion, the resin flows horizontally without anydeflection but in the recesses 128 a, 128 b corresponding to theportions of the take-up hub position regulating ribs 114 (see FIG. 8),the resin enters the inside thereof once, and thereafter, again returnsto move further forward. Also, at this time, the recesses 128 (128 a,128 b) are directed in a direction perpendicular to the advancedirection of the resin and the flow is further obstructed. Furthermore,if a gas that is generated due to heat is contained in the resin, theflow of the resin is further obstructed.

[0014] Thus, since the molten resin is filled to ride the portion of thetake-up hub position regulating ribs 114 (see FIG. 8), the fludizedlayer of the resin is advanced while being subject to the resistance. Asa result, the resin is spread and advanced more in the portion where notake-up hub position regulating ribs 114 are provided. As shown in aplan view of FIG. 11, the flows of the resin coming in the twodirections are brought into contact with each other at the end portionof the window portion 112 (the final filling portion of the resin). As aresult, an abnormal color portion having streaks that are called welds130 is formed at the joint portion to cause a problem in that the outerappearance becomes worse. In particular, since the window portion 112 istransparent, there is a problem in that such a quality reduction isundesirable.

[0015] By the way, as well known, in the magnetic tape cassette, underthe condition that the respective end portions of the longitudinalmagnetic tape are fixed to the pair of take-up hubs, respectively, thepair of take-up hubs around which the magnetic tape is wound areaccommodated in a cassette bodies (upper half and lower half). Only apart of the magnetic tape tensioned between the pair of take-up hubs isexposed to the outside of the cassette body such that the magnetic tapemay be brought into contact with a magnetic head at least when thecassette is loaded on a recording/reproducing apparatus of the magnetictape.

[0016] When the magnetic tape cassette is loaded on therecording/reproducing apparatus, the part of the magnetic tape tensionedbetween the take-up hubs and exposed from the cassette body is broughtinto contact with the magnetic head within the recording/reproducingapparatus, the take-up hubs are rotated, the magnetic tape travels andslidingly contacts with the magnetic head within therecording/reproducing apparatus, and the magnetic head recordsinformation onto the magnetic tape and reproduces the information fromthe magnetic tape.

[0017] In an audio magnetic tape cassette, when the magnetic tapecassette is loaded on the recording/reproducing apparatus, the part ofthe magnetic tape tensioned between the take-up hubs and exposed fromthe cassette body is brought into contact with the magnetic head withinthe recording/reproducing apparatus, the take-up hubs are rotated, themagnetic tape travels and slidingly contacts with the magnetic headwithin the recording/reproducing apparatus, and the magnetic headrecords information onto the magnetic tape and reproduces theinformation from the magnetic tape. In a cassette having a front coverfor protecting the tape such as a video magnetic tape cassette, when themagnetic tape cassette is loaded on the recording/reproducing apparatus,the front cover is rotated and moved to a position where the magnetictape is opened by a mechanism within the recording/reproducing apparatusin cooperation with the loading operation. The part of the magnetic tapetensioned between the take-up hubs is thus exposed from the cassettebody. Then, the take-up hubs are rotated, and the magnetic tape travelsand slides in contact with the magnetic head within therecording/reproducing apparatus, so that the magnetic head records theinformation onto the magnetic tape and reproduces the information fromthe magnetic tape.

[0018] When the magnetic tape moves up and down or meanders upon thetravel thereof, the magnetic tape is not exactly contacted with themagnetic head or slid well. Thus, the appropriate recording/reproducingis impossible. Also, there is a fear of a cause of other breakdown.Accordingly, it is necessary to suppress the up-and-down movement ormeanders upon the travel of the magnetic tape, to enhance the regulatingprecision of the travel position of the magnetic tape and to stable thetraveling property.

[0019] For this reason, conventionally, in the magnetic tape cassetteusing flangeless type take-up hubs, a pair of friction sheets areinterposed between the take-up hubs around which the magnetic tape iswound and the cassette bodies (upper and lower halves) to therebyregulate the travel position (height) of the magnetic tape to adjust thetravel position of the magnetic tape. Furthermore, a linear stampedportion (drawn portion) is provided on the friction sheet to regulatethe travel position (height) of the magnetic tape by the height of thedrawn portion.

[0020] On the other hand, when the thickness of the magnetic tape isequal to or less than 10 μm, depending upon the magnetic surface and thekind of the base material, the tape edge of the magnetic tape becomessharp so that the durability of the material that contacts with the tapeedge, such as the friction sheet causes a problem. Further, the surfaceelectric resistance value of the magnetic surface of the magnetic tapeand the base material is in the range of 1×10⁵ Ω to 1×10¹² Ω. There is aproblem of the fear of breakdown such as sticking of the magnetic tapeto the mechanism elements or an adverse affect to therecording/reproducing apparatus due to the electric charge caused by thefriction when the tape is entrained at a high speed. In particular, thelonger the length of the magnetic tape, the larger the effect thereofwill become. Therefore, there is a problem in that the tape is likely tobe electrically charged.

[0021] By the way, this friction sheet (hereinafter simply referred toas a sheet) is formed in general of high-density polyethylene resin(high density PE) or the like, and the electric resistance value thereofis very high at about 10¹⁶ Ω. For this reason, the sheet itself islikely to be charged. The sheet is readily charged and stuck with dustby the contact or the sliding contact of the sheet with the cassetteinner wall surface, the magnetic tape or the take-up hubs. The magnetictape or the take-up hubs are brought into sliding contact with the sheetto cause the peel charge so that this problem becomes more remarkable.

[0022] However, in the above-described conventional magnetic tapecassette, since the travel position of the magnetic tape is regulated bymeans of the friction sheet, the enhancement in the durability of thefriction sheet and the reduction in the surface electric resistancevalue should be resolved.

[0023] Here, the durability of the friction sheet means a cut of thetape edge. The cut of the tape edge may be solved by using a poroussheet as the sheet. Thus, the problem of the durability may be solved.

[0024] By the way, in order to reduce the surface electric resistancevalue of the friction sheet, by utilizing the fact that the sheet isporous, it is possible to consider that, for example, anti-static agentsuch as a mixture including quaternary ammonium salt is dipped or coatedto the sheet so that the anti-static agent is impregnated into thesheet. In the case where the anti-static agent is used for recoveringthe reduction of the surface electric resistance value of the sheet, thehigher the concentration of the anti-static agent, the lower the surfaceelectric resistance value will become, in general.

[0025] However, the anti-static agent is impregnated into fine holesformed in the sheet surface, fitted and adhered thereto. For thisreason, in some cases, the anti-static agent is peeled off and droppedoff by a slight collision. When the concentration of the anti-staticagent is increased, not only is the cost increased but also the amountof adhesion of the anti-static agent to the sheet is increased. As aresult, the amount of the anti-static agent that drops from the sheet isincreased. The drops adhere to the magnetic tape to contaminate the tapeand the interior of the cassette. The outer appearance and the productperformance are damaged to lead the reduction of the quality of themagnetic tape per se and of the magnetic tape cassette.

[0026] Incidentally, a variety of large capacity storage apparatuses areused as storage devices for a computer system. In view of the largecapacity of the storage, in particular, a magnetic tape medium isfrequently used by a large-scale user. Among these, recently, a cassettetype magnetic tape medium, i.e., a magnetic tape cassette is frequentlyused in view of the fact that it is easy to handle and to provide alarge capacity.

[0027] As such a magnetic tape cassette, one in which: (take-up) hubsaround which the magnetic tape is wound are accommodated on the rightand left sides within the case; an opening for drawing out the tape isformed on a front face of the case; the front lid for opening andclosing the opening is swingably installed on the front portion of thecase; and the a slider that may be slide back and forth is provided onthe lower surface side of the case, is used.

[0028] On the other hand, a cassette automatic delivery apparatus(hereinafter referred to as an auto loader) is an apparatus foraccommodating a plurality of magnetic tape cassettes and delivering thecassettes in order by a delivery mechanism and is connected such thatthe magnetic tape cassettes are continuously automatically fed to themagnetic head portion of the recording/reproducing apparatus.

[0029] As shown in FIG. 12, such an auto loader 140 is structured, forexample, as a recirculation type one such that magnetic tape cassettes81 are inserted into the upper stage of a frame body 142 formedgenerally into a box shape and are discharged from the lower stage.

[0030] In the apparatus exemplified in FIG. 12, seven magnetic tapecassettes 81 as a whole may be accommodated, two magnetic tape cassettes81 may be loaded on an upper stage guide member 144 formed in rails onthe side walls, three magnetic tape cassettes 81 may be overlapped in adeep accommodating portion 146, and furthermore two magnetic tapecassettes 81 may be accommodated in a lower stage delivery portion 148.

[0031] The two magnetic tape cassettes 81 inserted onto the upper stageguide member 144 move toward the deep accommodating portion 146, drop bytheir gravitational force, and overlapped in order as the next magnetictape cassette 81 is inserted. The lowermost magnetic tape cassette 81 ofthe deep accommodating portion 146 is delivered from the deep part tothe forehand side in the direction indicated by the arrows by a deliverymechanism (not shown) to be moved and discharged in order along thelower stage delivery portion 148.

[0032] In the lower stage delivery portion 148, the magnetic tapecassette 81 to be discharged is moved to depress the rear portion of anopposite case 82 to a front lid 91. Incidentally, the details of thedelivery mechanism and the opening portion of the frame member 142through which the delivery mechanism is entered or discharged are notshown in the figures.

[0033] In the above-described recirculation type auto loader 140, anupper wall 152 of the upper surface portion of the lower deliveryportion 148 is provided with a depression engagement member 150 forpositioning and stopping the magnetic tape cassette 81 to be fed out.This depression engagement member 150 is composed of a leaf spring 150 bprovided with a thin roller 150 a on a lower surface side, and one endof the leaf spring 150 b is fixed to the above-described upper wall 152as shown in FIG. 13.

[0034] The above-described roller 150 a is depressed and contacted withthe top surface of the case 82 of the magnetic tape cassette 81 by thebiasing force of the leaf spring 150 b. Then, a recess 85 is formed onthe front side of the top surface of the case 82 in the magnetic tapecassette 81. The roller 150 a is engaged with this recess 85 to therebyposition the cassette.

[0035] Incidentally, since the basic structure of the magnetic tapecassette 81 is the same as that of a magnetic tape cassette 80 shown inFIG. 6A and 6B to be described later except for a part (structure of theupper wall portion of a front lid 93 and the front end portion of anupper half 83) and the same reference symbols are used to indicate thesame or like components, see the explanation thereof. Also, in thefollowing description, the loading direction of the magnetic tapecassette 81 is referred to as the forward direction (direction indicatedby the arrow A in FIG. 13) and the direction opposite this is referredto as the backward direction.

[0036] However, as described above, in the case where the magnetic tapecassettes 81 are loaded on the recirculation type auto loader 140 havingthe structure shown in FIG. 12 and delivered in order, when the roller150 a of the depression engagement member 150 for positioningcorresponding to the delivery movement is moved under the depressioncondition on the top surface of the magnetic tape cassette 81, theabove-described roller 150 a is hooked to the front lid 91 to become anobstacle against the stable delivery movement and to apply anunnecessary (extra) force to the front lid 91 to thereby cause thecurvature deformation to depress and open the front lid 91 and tothereby damage it.

[0037] Namely, as shown in FIG. 13, when the roller 150 a depressed bythe biasing force of the leaf spring 150 b is engaged with the recess 85on the upper surface of the case 82 of the magnetic tape cassette 81,the magnetic tape cassette 81 is moved rightward in FIG. 13 (in thedirection indicated by the arrow D) by the delivery mechanism, theroller 150 a is moved relative to the forward side on the top surface ofthe case 82 from the recess 85 and enters into a gap 98 defined betweenthe rear end portion of the upper portion of the front lid 91 and thefront end portion of the top surface of the case 82 as shown byreference symbol 150 a indicated by the broken lines. In some cases, theroller 150 a is hooked at the rear end of the top portion of the frontlid 91 and elastically deformed such that the central portion of a lidportion 92 is curved forward by means of the delivery force to besubjected to the force of opening (denoted by 92′ indicated by anotherbroken line in FIG. 13). In particular, the central portion of theabove-described front lid 91 has a low rigidity and is deformed even bya weak force to expand the gap 98, and the above-described roller 150 ais likely to be hooked.

[0038] Incidentally, the depression force of the above-described roller150 a is about 1 to 2 N. This is sufficient to depress and position asmall size magnetic tape cassette 81. However, the roller 150 a ishooked at the front lid 91 as described above and causes the generationof the opening force. If the depression force of the roller 150 a isreduced, the generation of the hook at the above-described front lid 91may be suppressed but there is a problem in that the positioning forceis insufficient and unstable.

SUMMARY OF THE INVENTION

[0039] In view of the above-described problems inherent in theconventional art, a primary object of the present invention is toprovide a cassette half and a molding method therefor that may preventthe generation of a weld in a window portion of the cassette half and toprevent the degradation in molding outer appearance in a cassette halfwhose window portion and upper half body is to be molded in a two-colorinjection molding.

[0040] In view of the above-described problems inherent in theconventional art, a secondary object of the present invention is toprovide a magnetic tape cassette that may reduce an amount of wear orcut of a magnetic tape edge per se and a friction sheet while preventingthe sticking of the magnetic tape without degrading the anti-staticeffect to the friction sheet and may remarkably reduce an amount of dropof an anti-static agent due to the friction even if the magnetic tapehaving the sharp tape edge is used and the friction between the magnetictape edge and the sheet is kept at a high speed.

[0041] In view of the above-described problems inherent in theconventional case, in order to overcome the problems, a third object ofthe present invention is to provide a magnetic tape cassette in which adetecting roller of an auto loader is prevented from being hooked at afront lid of the magnetic tape cassette so that the front lid is notsubjected to the opening force of opening the front lid upon thedelivery by the auto loader.

[0042] In order to the primary object described above, the first mode ofthe first aspect of the present invention provides a cassette halfmolding method for molding one of upper and lower halves with aninjection mold by injection molding, the upper and lower halves beingcoupled with each other in a lid alignment manner to form a cassettecase for accommodating a pair of take-up hubs around which a magnetictape is wound, comprising the step of: molding a transparent orsemitransparent window portion provided in the upper half with theinjection mold, wherein a gas vent pin for smoothing a flow of moltenresin to be introduced in the window portion is provided in theinfection mold.

[0043] In order to the primary object described above, the second modeof the first aspect of the present invention provides a cassette halfthat is molded with an injection mold by injection molding and that isone of upper and lower halves that are coupled with each other in a lidalignment manner to form a cassette case for accommodating a pair oftake-up hubs around which a magnetic tape is wound, wherein atransparent or semitransparent window portion having a take-up hubposition regulating rib is provided on the upper half, and wherein theupper half is molded by the infection mold provided with a gas vent pinfor smoothing a flow of molten resin at a portion perpendicular to aflow direction of the molten resin of a ceiling surface of the take-uphub position regulating rib when the window portion is molded by theinjection molding.

[0044] In order to the secondary object described above, the secondaspect of the present invention provides a magnetic tape cassettecomprising: a body formed by coupling an upper half and a lower halfwith each other in a lid alignment manner; a pair of take-up hubs aroundwhich a magnetic tape is wound and which are rotatably accommodated inthe cassette body; and a friction sheet interposed either between thepair of take-up hubs and the upper half, or between the pair of take-uphubs and the lower half, or both, wherein the thickness of the magnetictape is equal to or less than 7.7 μm, the maximum winding length isequal to or more than 120 m, and the maximum speed for the tape beingwound to the cassette body is 2 m/sec or more, and wherein the frictionsheet is made of polyethylene-based resin and has a surface electricresistance value in the range of 1×10⁷ Ω to 1×10¹⁰ Ω.

[0045] It is preferable that the polyethylene-based resin isfluorine-based resin in which a part of or all of hydrogen atomscontained in molecules constituting a polymer compound are substitutedby fluorine atoms.

[0046] In order to the third object described above, the third aspect ofthe present invention provides a magnetic tape cassette which is loadedonto a cassette auto delivery apparatus for accommodating a plurality ofmagnetic tape cassettes and for sequentially delivering the magnetictape cassette by a delivery mechanism, comprising: a body case; a firstrecess formed in a top surface of the body case for retaining adepression engagement member having a spring and a roller provided inthe cassette auto delivery apparatus for positioning and stopping; afront lid provided swingably at a front face of the body case foropening and closing an opening for drawing out a magnetic tape; and aslider disposed in a lower surface of the body case and slidable inbackward and forward directions, wherein a rib for reducing a resistancewhen the roller passes through a gap between the top surface of the bodycase and the front lid is provided at a position corresponding to thefirst recess and on a side of the front lid in the top surface of thebody case.

[0047] It is also preferable that the magnetic tape cassette furthercomprises a second recess corresponding to a convex portion of the ribat a position of the front lid corresponding to the rib.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In the accompanying drawings:

[0049]FIG. 1 is a cross-sectional view showing a state in which a windowportion of an upper half in accordance with an embodiment of the presentinvention through a molding method of a cassette half in accordance withthe present invention;

[0050]FIG. 2 is a perspective view showing simply a state of molding ofthe window portion shown in FIG. 1;

[0051]FIG. 3 is an exploded perspective view of an embodiment of thecassette body of the magnetic tape cassette according to the invention;

[0052]FIG. 4 is a partially exploded perspective view showing aschematic structure of an embodiment of a magnetic tape and frictionsheets for clamping on both sides the magnetic tape wound around pair oftake-up hubs which are accommodated in the cassette body of the magnetictape cassette shown in FIG. 3;

[0053]FIG. 5 is a graph showing a relationship between the surfaceelectric resistance value of the friction sheet and the rate of stickingbetween the friction sheet and the magnetic tape and the relationship ofthe peel rate of the anti-static agent;

[0054]FIG. 6A is a schematic perspective view of an embodiment of amagnetic tape cassette according to the present invention;

[0055]FIG. 6B is an enlarged detailed partially cutaway perspective viewof B portion shown in FIG. 6A;

[0056]FIG. 7 is a cross-section view of the magnetic tape cassette takenalong the line VII-VII of FIG. 6;

[0057]FIG. 8 is a plan view of an upper half of a conventional magnetictape cassette;

[0058]FIG. 9 is a cross-sectional view taken along the line IX-IX ofFIG. 8;

[0059]FIG. 10 is a cross-sectional view showing a conventional moldingmethod of the upper half;

[0060]FIG. 11 is a plan view of a window portion of the upper halfshowing a problem inherent in the conventional molding method;

[0061]FIG. 12 is a view showing a schematic structure of an auto loader;and

[0062]FIG. 13 is a view showing a concept of a magnetic tape cassettedetection mechanism in the auto loader.

DETAILED DESCRIPTION OF THE INVENTION

[0063] A magnetic tape cassette, a cassette half and a cassette halfmolding method according to the present invention will now be describedin detail with reference to the accompanying drawings.

[0064] A cassette half and its molding method in accordance with a firstaspect of the present invention will now be described with reference toFIGS. 1 and 2.

[0065] A state is shown in FIG. 1 in which an upper half having atransparent window portion according to a second embodiment of thepresent invention is formed in a two-color injection molding inaccordance with the molding method of the cassette half in accordancewith a first embodiment of the first aspect of the present invention.

[0066] As shown in FIG. 1, an upper half 10 of a DDS (Digital DataStorage) cassette or cartridge has a transparent window 12 and is moldedby means of molding dies (a front mold 20 a, a movable cavity or mold 20b and a slide core 22). In the same manner as in the conventional case,first of all, molten resin is introduced into a space (cavity)surrounded by the front mold 20 a, the movable mold 20 b and the slidecore 22.

[0067] Thereafter, as shown in FIG. 1, the slide core 22 is slidinglymoved corresponding to a thickness of the window portion 12 to form thespace (cavity) 24 for molding the window portion 12. The transparentresin (for example, AS) is injected from a gate 26 to the cavity 24 toform the window portion 12.

[0068] At this time, the molten resin injected from the gate 26 into thecavity 24 is advanced as indicated by the arrows in FIG. 1. The resin isonce introduced into recessed portions such as recesses 28 (28 a, 28 b)of the slide core 22 corresponding to take-up hub position regulatingribs 14 (14 a, 14 b) and again comes out to flow straightly.

[0069] At this time, according to this embodiment, a gas vent pin 30 forsmoothing the resin fluid is provided in a ceiling surface 15 of thetake-up hub position regulating rib 14 b on a remote position from thegate 26. Since the gas vent pin 30 is provided as a discrete member inthe slide core 22, a slight gap (clearance) 30 a is provided between theslide core 22 and the gas vent pin 30 so that the gas generated in theresin due to the heat will be discharged therefrom.

[0070] This state is shown in a perspective view of FIG. 2. In FIG. 2,for the sake of simplification, only the gate and the gas vent pin areshown.

[0071] As shown in FIG. 2, the gas vent pin 30 is provided in an uprightposition on the ceiling surface 15 of the take-up hub positionregulating rib 14 b on a remote position from the gate 26. Furthermore,the resin flows from the gate 26 on the right and left sides (in thelongitudinal direction of the window portion 12). The gas vent pin 30 isprovided at the position perpendicular to the direction in which theresin flows. As shown in FIG. 2, the window portion 12 is symmetric withrespect to the gate 26. In the same manner, a gas vent pin 32 is alsoprovided in a ceiling surface of a rib 16 b at a remote position fromthe gate 26 of the other take-up hub position regulating ribs 16 (16 a,16 b) provided in the window portion 12.

[0072] As described above, in the portion of the take-up hub positionregulating ribs 14, 16 that are provided in the window portion 12, theflow of the molten resin is changed and the flow is obstructed. At thistime, the gas generated within the molten resin is stagnant in therecesses or recessed portions, where such a molten resin flow ischanged, to thereby further obstruct the flow of the molten resin.

[0073] In this embodiment, since the gas vent pins are provided at thepositions where the resin flow is stagnant (changed), it is possible todischarge the gas to the outside of the molds effectively and to smooththe resin flow. As a result, it is possible to suppress the generationof the weld.

[0074] Incidentally, in this embodiment, the gas vent pins are providedin the ceiling surfaces of the take-up hub position regulating ribsremote from the gate. Since the resin is just injected from the gate andthe pressure of the resin is still kept high on the near side to thegate, the obstruction against the flow is relatively small. Therefore,it is more effective to provide the gas vent pins on the remote sidefrom the gate.

[0075] However, it goes without saying that in the same manner the gasvent pins may be provided also on the take-up hub position regulatingribs on the near side to the gate. The gas vent pins are provided in thepositions of the ceiling surfaces of both the take-up hubposition-regulating ribs to thereby ensure the further effect.

[0076] Thus, it is preferable to provide the gas vent pins to thepositions of the ceiling surface of the take-up hub position regulatingribs remote from the gate. However, the position is not limited thereto.The gas vent pins are provided in the position where the resin flow ischanged to ensure the sufficient gas ventilation effect.

[0077] As described above, in this embodiment, since the gas vent pinsfor discharging the gas that obstructs the flow of the molten resin areprovided in the most effective positions, it is possible to dischargethe gas to the outside of the molds effectively to smooth the flow ofthe resin to thereby prevent the generation of the weld and to therebygive a good appearance to the final product of the upper half.

[0078] The cassette half and the molding method thereof according to thefirst aspect of the present invention are constructed basically asdescribed above.

[0079] A magnetic tape cassette according to a second aspect of thepresent invention will now be described with reference to FIGS. 3 and 4.

[0080]FIG. 3 is a partially exploded perspective view showing aschematic structure of one embodiment of a cassette body of a magnetictape cassette according to a second aspect of the present invention.FIG. 4 is a partially exploded perspective view showing a schematicstructure of one embodiment of a magnetic tape and friction sheets to beaccommodated in the cassette body of the magnetic tape cassette shown inFIG. 3, the friction sheets clamping on both sides the magnetic tapewound around a pair of take-up hubs.

[0081] As shown in these drawings, the magnetic tape cassette(hereinafter simply referred to as a cassette) 40 is formed into a boxby alignment and abutment of halves and has a pair of an upper half(upper cassette half) 42 and a lower half (lower cassette half) 44 (seeFIG. 3) constituting the cassette body, a pair of take-up hubs 46, 46 tobe accommodated in the cassette body, a magnetic tape 48 to be woundaround the pair of take-up hubs 46, 46, and friction sheets (hereinaftersimply referred to as sheets) 50, 50 (see FIG. 4) interposed between thetake-up hubs 46, 46 and the magnetic tape 48 to be wound around the pairof these hubs and the upper half 42 and between the take-up hubs 46, 46and the magnetic tape 48 to be wound around the pair of take-up hubs 46,46 and the lower half 44, respectively.

[0082] As shown in FIG. 3, the pair of upper half 42 and lower half 44have substantially the same shape and are provided with a partitioningwall 56 for partitioning the hollow interior portion of the cassettebody, formed by these halves, into a recording/reproducing region 52 anda hub accommodating region 54 (where the interior portion is not shownin the upper half 42).

[0083] A pair of spindle shaft insertion holes 58, 58 into which a pairof spindle shafts of a recording/reproducing apparatus for driving androtating the pair of take-up hubs 46, 46 are to be inserted, a pair ofhub supporting annular ribs 58 a, 58 a for pivotally regulating (orsupporting) the pair of take-up hubs 46, 46 along the innercircumferential edges of these spindle shaft insertion holes 58, 58, apair of rotary guide rollers 60, 60 on both sides on the front side andthe like are provided substantially in the central portion on both sidesof the hub accommodating region 54 of each of the pair of upper andlower halves 42 and 44 symmetrically on right and left sides withrespect to the centerline in the cassette front and rear direction(where the side of the recording/reproducing region 52 to the hubaccommodating region 54 will hereinafter be referred to as a frontside). Incidentally, the pair of guide rollers 60 and 60 are rotatablysupported to a pair of pins 60 a and 60 a implanted in an uprightposition in the lower half 44.

[0084] Also, a pair of capstan insertion holes 62, 62, a pair ofcassette positioning holes 64, 64 and the like are formed symmetricallywith respect to the above-described centerline in therecording/reproducing region 52 of each of the pair of upper and lowerhalves 42, 44. A pair of head pins 66, 66 and the like are provided inan upright position symmetrically with respect to the above describedcenterline on the inside of the pair of cassette positioning holes 64,64 in the recording/reproducing region 52 of the lower half 44.

[0085] Furthermore, a leaf spring 70 that is a planar support member forsupporting a pressure pad 68 is provided on the rear side of the pair ofhead pins 66, 66, and a shield plate 72 is assembled on the rear side ofthe leaf spring 70. A magnetic head insertion opening 74 through whichthe magnetic head (not shown) of the recording/reproducing apparatus isto be inserted from the front side of the cassette upon therecording/reproducing mode is formed in the front side of the centralportion of the recording/reproducing region 52 of each of the pair ofupper and lower halves 42 and 44.

[0086] Then, when the magnetic head is inserted into therecording/reproducing region 52 from the magnetic head insertion opening74, a tip end of the magnetic head and the magnetic tape 48 is depressedinwardly of the cassette 40 together with the pressure pad 68. As aresult, the magnetic tape 48 is brought into pressing contact with themagnetic head (head gap formed at its tip end) by the tension of themagnetic tape 48 tensioned between the pair of head pins 66, 66 and thedepression force of the pressure pad 68 on the basis of the biasingforce of the leaf spring 70. In the recording/reproducing mode, thetake-up hubs 46, 46 engaging with the pair of spindle shafts of therecording/reproducing apparatus are driven and rotated by means of thespindle shafts. The magnetic tape 48 is tensioned between both head pins66, 66 and travels in sliding contact with the magnetic head. The audioinformation recorded on the magnetic tape is reproduced, and inversely,the audio information is recorded on the magnetic tape.

[0087] As shown in FIG. 4, the magnetic tape 48 is wound around the pairof take-up hubs 46, 46. Each of the take-up hubs 46, 46 is provided atits outer circumference with the tape winding ring 46 a. The sidesurface becomes the magnetic tape winding surface. Also, a small piece47 that may be removed for fixing the end portion of the magnetic tape48 is mounted on the part of the tape-winding ring 46 a. A smalldiameter ring 46 b is formed on the inner circumferential side of thetake-up hub 46 and engages with the hub supporting annular rib 58 a ofthe spindle shaft insertion hole 58 of the pair of upper and lowerhalves 42 and 44 to support the take-up hub 46 rotatably and to regulatethe removal of the take-up hub 46 from the position of the spindle shaftinsertion hole 58.

[0088] Here, the magnetic tape 48 used in accordance with the inventionhas to have a maximum winding length of 120 m or more and a thickness of7.7 μm or less. Also, the magnetic tape 48 has a maximum winding speedof 2 m/sec or more to the cassette body of the cassette 40. Namely,there is a case where the magnetic tape 48 used in the present inventionhas the winding speed of 2 m/sec or more when the magnetic tape 48 iscut by, for example, a slitter to be wound into the cassette body uponthe manufacture of the cassette 40 according to the present invention.Also, there is a case where the winding speed of the magnetic tape intothe cassette body in the fast feed mode or the rewind mode when themagnetic tape is mounted on the recording/reproducing apparatus is 2m/sec or more. Incidentally, it goes without saying that the windingspeed of the magnetic tape into the cassette body when the cassette isloaded on the recording/reproducing apparatus for the usualrecording/reproducing mode, i.e., the regular recording or reproducingspeed may be 2 m/sec or less and usually, equal to 2 m/sec or less.

[0089] It is preferable that the thickness of the magnetic tape 48 is inthe range of 2 to 6 μm, the maximum winding length is in the range of120 to 150 m, and the maximum winding speed to the cassette body is inthe range of 2 to 4 m/sec.

[0090] The reason why the thickness of the magnetic tape 48 used in thepresent invention is limited to 7.7 μm or less is that the tape edge ofthe magnetic tape having the thickness of 7.7 μm or less becomes sharp,and the durability of the member that is to come into contact with tapeedge, i.e., a friction sheet 50 to be described later would cause aproblem. Also, the reason why the maximum winding length of the magnetictape 48 is limited to 120 m or more and the maximum winding speed of themagnetic tape 48 to the cassette body is limited to 2 m/sec or more isthat the surface electric resistance value of the magnetic surface orthe base material of the regular magnetic tape 48 that used in thepresent invention is in the range of 1×10⁵ Ω to 1×10¹² Ω, there is anadverse affect to the head of the recording/reproducing apparatus andthe breakdown such as the sticking of the magnetic tape to themechanical parts due to the charge caused by the friction when the longtape is wound, or when the tape is wound at a high speed. In particular,the longer the magnetic tape, the more the adverse affect will becomeand the more likely the charging will be.

[0091] The magnetic tape 48 used in the present invention has theabove-described thickness and length. There is no problem if the windingspeed to the cassette body may be the above-described maximum windingspeed and any kind of magnetic tape may be used. Namely, there is nospecial limit put on the material and use of the magnetic tape of thecassette according to the present invention. Accordingly, there is nospecial limit put on the base material or the magnetic material used.The base material or magnetic material may be made of any kind ofmaterial.

[0092] Also, it is possible to select any kind of known magneticmaterial used in the conventional magnetic tape as the magnetic materialof the magnetic tape to be used in the present invention. It is possibleto select as the magnetic tape used in the present invention themagnetic tape in which a magnetic material and a base material arecontacted with each other and cut in a certain length. Among these,after the slit process, it is preferable to use the magnetic tape thathas not been subjected to any special coating.

[0093] Also, it is possible to select as the base material of themagnetic tape used in the present invention any known base material usedin the conventional magnetic tape. However, it is preferable to selectas the base material PET (polyethylene terephthalate), PEN (polyethylenenaphthalate), aramide or the like. It is preferable that the thicknessof the magnetic material is in the range of 2 to 6 μm. More preferably,the thickness is in the range of 2.5 to 4 μm. Also, it is preferablethat Young's coefficient of the magnetic tape is 500 kg/mm² or more inthe lateral and longitudinal directions.

[0094] Also, as shown in FIG. 4, the friction sheets 50, 50 areinterposed between the take-up hubs 46, 46 around which the magnetictape 48 is wound and the inner wall surface of the cassette 40, i.e.,between the take-up hubs 46, 46 and the upper half 42 and between thetake-up hubs 46, 46 and the lower half 44, respectively. Four smallholes 50 a formed in these sheets 50, 50 are engaged with four ribs 42 aof the upper half 42 and the lower half 44 shown in FIG. 3, so that thesheets 50 are positioned in the upper half 42 and the lower half 44.Also, pairs of insertion holes 50 b, 50 b are formed in the sheet 50, 50corresponding to the respective two spindle shaft insertion holes 58, 58of the upper half 42 and the lower half 44.

[0095] Two linear stamped portions two drawn portions 50 c and 50 d areprovided in the respective cassette right and left directions (in thedirection perpendicular to the back-and-forth direction of the cassette40) in the friction sheets 50, 50. The sheets 50, 50 serve to reduce thesliding resistance when the take-up hubs 46, 46 rotate and to smooth therotation and at the same time to support the take-up hubs 46, 46 aroundwhich the magnetic tape 48 is wound and to regulate the height by thestamped portions 50 c, 50 d.

[0096] In the foregoing example, the sheets 50, 50 are interposedbetween the magnetic tape 48 and both the pair of take-up hubs 46, 46and the upper half 42 and the lower half 44, respectively. However, thepresent invention is not limited to this specific arrangement. The sheetmay be interposed only between the hubs and either one of the halves.

[0097] It is necessary to make the friction sheet 50 ofpolyethylene-based resin according to the present invention. This isbecause the sheet is not worn or less to keep the durability of thesheet by the contact at the tape edge even in the magnetic tape 48having the sharp tape edge having a thickness of 7.7 μm or less. Sincethe durability of the sheet 50 may be further enhanced, it is morepreferable that the sheet 50 is made of porous material.

[0098] The friction sheet 50 according to the present invention has thesurface electric resistance value in the range of 1×10⁷ Ω to 1×10¹⁰ Ω.The surface electric resistance value needed for the sheet 50 withrespect to the magnetic tape having the sharp tape edge that might beused in the above-described circumstances has been reviewed as a resultof which this range is preferable.

[0099] The surface electric resistance value of the sheet 50 is selectedin the above-described range, whereby, even if the friction between thesheet and the magnetic tape having the sharp edge is kept at a highspeed, the amount of drop of the anti-static agent due to the frictionis suppressed to a very low level, and it is possible to prevent thesticking of the magnetic tape without degrading the anti-static effectto the friction sheet and to suppress the wear of the sheet and themagnetic tape edge per se. Incidentally, the preferable range of thesurface electric resistance value is in the range of 1×10⁹ Ω to 1×10¹⁰Ω.

[0100] In the present invention, in the case where the magnetic tapecassette has a plurality of friction sheets, it is sufficient that oneout of these sheets meets the above-described requirements.

[0101] In order to meet the requirements described above, it is possibleto impregnate the anti-static agent having a suitable concentration tothe friction sheet. The anti-static agent to be used is not limited toany special one. For example, it is possible to use mixture containingquaternary ammonium salt, or the like.

[0102] As the impregnating method of the anti-static agent into thefriction sheet, it is possible to adopt, for example, a dipping methodof dipping an original friction sheet into an anti-static agent bath, amethod of coating with a so-called coater and so on. It is possible toperform a well-known plasma process, a corona discharge process, anultraviolet ray process or the like in advance to enhance thecharacteristics of the sheet in advance.

[0103] Here, as a method of measuring the surface electric resistancevalue of the magnetic tape and the friction sheet, a method inaccordance with JIP X 6130 was adopted.

[0104] The reason why the friction sheet to be used in the magnetic tapecassette according to the present invention has to meet thecharacteristics as described above will now be described with referenceto FIG. 5.

[0105]FIG. 5 is a graph showing an example of a relationship between thesurface electric resistance value of the friction sheet and the rate ofsticking of the magnetic tape to the friction sheet, and therelationship between the surface electric resistance value and the peelrate of the anti-static agent.

[0106] A friction sheet made of porous polyethylene was prepared as thefriction sheet and was impregnated on both side surfaces by the dippingmethod using an anti-static agent (ammonium salt mixture) of variousconcentrations so that the surface electric resistance value was in therange of 1×10⁵ Ω to 1×10¹³ Ω.

[0107] Next, a magnetic tape was prepared in which a base material wasaramide and wherein the thickness of the magnetic layer was 2 μm, thetotal tape thickness was 5.6 μm, the width was 4 mm, and the surfaceelectric resistance value was 1.0×10⁶ Ω.

[0108] Samples of the cassette bodies provided with the above describedvarious friction sheets were prepared. Under the condition oftemperature of 23° C. and the relative humidity of 50%, theabove-described magnetic tape was wound in the cassette bodies at aspeed of 1 to 4 m/sec by 125 m. Thereafter, the rate of sticking of themagnetic tape (%) and the peel rate of the anti-static agent impregnatedinto the friction sheet were measured. The results are shown in FIG. 5.Incidentally, the rate of sticking of the magnetic tape is shown by ahollow circle. Also, the peel rate of the anti-static agent wasindicated by solid signs by obtaining the result through the testsconducted eight times for each surface electric resistance value.

[0109] Here, the magnetic tape sticking means the phenomenon in whichthe magnetic surface of the magnetic tape is stuck to a certain surfaceof the cassette or the upper and lower sheets. Also, the magnetic tapesticking rate (%) means the proportion of cassettes in which thesticking takes place in the sampled cassette bodies (5,500 samples).

[0110] Also, the peel test of the anti-static agent was performed byremoving the magnetic tape from the cassette body after the winding ofthe magnetic tape and observing with human eye whether or not theresidue of the anti-static agent was stuck to the magnetic tape edge.Also, the peel rate (%) means the proportion of cassettes in which theresidue was found on the magnetic tape edge in the sampled cassettebodies (100 samples) Incidentally, the judgement as to whether or notmaterials adhered to the magnetic tape edge were the anti-static agentwas made through an FT-IR transmission method.

[0111] As shown in FIG. 5, when the surface electric resistance valuewas 1×10⁷ Ω to 1×10¹⁰ Ω, it was possible to suppress the tape stickingrate down to 0.02% or less and to suppress the peel rate of theanti-static agent down to 46% or less.

[0112] For this reason, it is understood that the use of the frictionsheet in these ranges makes it possible to suppress the peel of theanti-static agent to the minimum level and to obtain the magnetic tapecassette in which the magnetic tape would not be stuck to the frictionsheet without degrading anti-static effect of the friction sheet.

[0113] Here it is possible to select, as the polyethylene-based resinused in the manufacture of the friction sheet 50 according to thepresent invention, polyethylene (PE) resin, porous polyethylene resin,polyethylene terephthalate (PET), or the like.

[0114] Furthermore, it is preferable to select, as thepolyethylene-based resin used in the sheet 50 according to the presentinvention, fluorine-substituted polyethylene resin where a part of orall of hydrogen atoms contained in molecules of a polyethylene-basedresin composition are substituted by fluorine atoms. It is possible toselect, as such a fluorine-substituted polyethylene-based resin,polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene copolymer(ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP),polyflruorovinylidene (PVDF), or the like.

[0115] Thus, the use of the fluorine-substituted polyethylene-basedresin as the polyethylene-based resin for manufacturing the sheet 50according to the present invention makes it possible to reduce thedynamic frictional coefficient of the sheet 50 and to enhance thesliding property of the magnetic tape. For instance, the value of thedynamic friction coefficient μ of the PE resin is approximately in therange of 0.22 to 0.35 whereas the dynamic friction coefficient μ of thePTFE resin is approximately in the range of 0.20 to 0.27. Accordingly,the PTFE resin can be used instead of the PE resin to thereby reduce thedynamic friction coefficient and to enhance the sliding property of themagnetic tape.

[0116] Examples of use of the magnetic tape cassette according to thepresent invention include known use as an audio magnetic tape cassette,a video magnetic tape cassette, a computer backup magnetic tapecassette, etc. Namely, the magnetic tape cassette according to thepresent invention is not limited to the above-described audio magnetictape cassette but can be applied to, for example, a magnetic tapecassette that is of a type having a front lid for protecting the exposedfront magnetic tape such as a video (VHS) magnetic tape cassette or aneight millimeter cassette, or a magnetic tape cassette that has a frontlid or a slider for protecting the magnetic tape exposed on the frontside such as a digital audio tape cassette (DAT) or a digital datastorage (DDS), or the like.

[0117] Incidentally, in the cassette such as a video magnetic tapecassette, a digital audio tape cassette and a digital data storage inwhich the form of use is determined in advance, it is preferable for thefriction sheet for supporting the take-up hubs and the magnetic tapefrom below, namely, the lower friction sheet to be formed of a porouspolyethylene-based resin having the surface electric resistance valuemeeting the above-described characteristics, i.e., in the range of 10⁷ Ωto 1×10¹⁰ Ω. It is even more preferable to form the sheet from afluorine-substituted polyethylene resin sheet that meets theabove-described characteristics. Since the friction sheet located on thelower side has greater frictional force than that of the upper sheet andgreater sliding force than that of the upper sheet, such sheet is usedas the sheet of the present invention to thereby prevent the sticking ofthe magnetic tape to the sheet and reduce the adverse affect of the dropof the anti-static agent.

[0118] The magnetic tape cassette according to the second aspect of thepresent invention is constructed basically as described above.

[0119] A magnetic tape cassette in accordance with a third aspect of thepresent invention will now be described with reference to FIGS. 6A, 6Band 7.

[0120]FIG. 6A is a schematic perspective view of a magnetic tapecassette 80 in accordance with one embodiment of the present invention.FIG. 6B is an enlarged detailed partially cutaway perspective view of Bpart shown in FIG. 6A. FIG. 7 is a cross-sectional view taken along thevertical surface along the moving direction of the magnetic tapecassette 80 passing through a central portion of its recess 85.

[0121] As described above, the magnetic tape cassette 80 is composed ofa flat box-shaped case 82 obtained by coupling an upper half 84 and alower half 86, a front lid 90 openably, closably and swingably mountedonto the front end portion of this case 82 and a slider 88 slidablymounted on the lower surface side of the case 82.

[0122] Although not shown, the magnetic tape is wound around both hubsand accommodated in the interior of the case 82. The front bottomsurface portion and the front surface portion of the case 82 are cut outand an opening for tape loading for the recording/reproducing apparatusis formed. The lock of the front lid 90 that may open and close theopening for the tape drawing on the front surface side of the case 82 isreleased, and operates to open and close in accordance with a movingdistance of the slider 88 when the slider 88 is moved backward in timewith the loading operation onto the recording/reproducing apparatus.

[0123] Also, the slider 88 opens and closes the bottom opening portionof the lower half 86 in accordance with the back-and-forth slidingmovement along the loading direction A as described above. Incidentally,drive shaft insertion holes are opened on both sides of the lower half86 and the slider 88. Furthermore, a lock mechanism is provided forlocking the slider 88 in the open position and the closed position. Theabove-described lock mechanism is released in accordance with theloading operation to the recording/reproducing apparatus. The slider 88is then moved backward to open the bottom surface opening to lock theopen position in the rearward position. A tape-loading device isinserted into the opened opening portion to draw the tape forward and tomake it possible to open the front cover 90 for therecording/reproducing operation.

[0124] In the thus constructed magnetic tape cassette 80, the recess 85is formed substantially in the central portion of the front portion ofthe upper surface of the upper half 84. The roller 150 a of depressionengagement member 150 for positioning in the auto loader 140 shown inFIG. 12 forehand is engaged with the recess 85.

[0125] Incidentally, the above-described recessed portion 85 is alsocalled a changer grip.

[0126] The front lid 90 has a lid portion 92 for covering the frontsurface portion of the case body 82 and side walls 94 extending in theback and forth direction and coupled with both end portions thereof. Apair of pivot portions 94 a of the side walls 94 on both sides arepivotally supported to the side surfaces of the upper half 84. Theabove-described lid portion 92 has an upper wall portion 96 bentrearward at its upper end portion. In the closed state shown in FIG. 6,the upper wall portion 96 is constructed so as to be continuous with thetop surface of the upper half 84.

[0127] As shown in FIG. 7, a roller outlet side of the top surface ofthe above-described half 84 (on the side of the front cover 90) is aslanted flat surface (taper surface) 84 a. A rib-shaped convex portion(hereinafter referred to as a rib) 84 b is provided in the centralportion in the widthwise direction of the taper surface 84 a. This rib84 b is formed to allow the roller 150 a to pass smoothly through thegap between the upper wall portion 96 of the front lid 90 and theabove-described upper half 84.

[0128]FIG. 7 is an enlarged view of the vicinity of the gap 98 betweenthe upper wall portion 96 of the front lid 90 and the above-describedupper half 84. As shown in FIG. 7, since the bottom of theabove-described gap 98 is made shallow by the rib 84 b provided on theupper half 84, it is possible to realize the smooth passage of theroller 150 a through the gap 98 in the magnetic tape cassette 80according to this embodiment. It will be well understood that thepassage of the roller 150 a is facilitated in comparison with the caseof the same portion of the conventional structure (where the rib 84 b isnot formed in the above-described gap 98).

[0129] Here it is preferable that the height of the above-described rib84 b is approximately in the range of 0.2 mm to 0.7 mm. Also, the widthof the above-described rib 84 b is sufficient to be somewhat wider thanthe width of the roller 150 a, and for instance may be the widthcorresponding to the recess 85.

[0130] A recess (recessed portion) 96 b having a depth and a widthcorresponding to the above-described rib 84 b is formed in a position ofthe upper wall portion 96 of the above-described front lid 90corresponding to the above-described rib 84 b. The recess 96 b thussuppresses the reduction in strength of the upper wall portion 96 of thefront lid 90 to a minimum level.

[0131] Also, the front end portion of the upper wall portion 96 of theabove-described front lid 90 (upper half 84 side) is formed into a tapersurface 96 a slanted toward the upper half 84. The slant angle of thistaper surface 96 a is preferably at 45 degrees for instance. However,this is not limited thereto or thereby. The angle may be in the range ofabout ±15 degrees with respect to 45 degrees. Furthermore, the shape ofthe taper surface 96 a is not limited to the flat shape but the form ofthis taper surface 96 a can also preferably be selected from varioussecondary curved surfaces such as a circle, an oval shape, a hyperbolicshape, and a parabolic shape. Also, a combination of a flat surface anda curved surface and a combination of different curved surfaces may beused.

[0132] According to the above-described embodiment, in the magnetic tapecassette 80, it is possible to facilitate the passage of the roller 150a of the auto loader through the gap 98 between the upper wall portion96 of the front lid 90 and the above-described upper half 84 and toperform the smooth delivery of the magnetic tape cassette 80 in the autoloader. Thus, it is possible to obtain the effect for protecting themagnetic tape cassette 80 from the accident such as a breakdown.

[0133] One example of the present invention is shown in the form of theabove-described embodiment. It goes without saying that the presentinvention is not limited thereto or thereby.

[0134] For example, it is possible to provide a taper surface like thetaper surface 96 a provided on the side of the recess 85 of the upperwall portion 96 of the front lid 90 at the outlet side of the recess 85of the top surface of the above-described half 84. Otherwise, the sideof the recess 85 of the upper wall portion 96 of the front lid 90 may bemachined rough.

[0135] The magnetic tape cassette according to a third aspect of thepresent invention is constructed basically as described above.

[0136] Thus, the magnetic tape cassette, the cassette half used thereinand the method for molding the cassette half have been described indetail while showing various embodiments. The present invention is notlimited thereto or thereby. It goes without saying that variousmodifications and changes are possible within the scope of the presentinvention without departing from the spirit of the present invention.

[0137] As described in detail above, according to the cassette half andits molding method of the first aspect of the present invention, it ispossible to discharge the gas contained in the molten resin generateddue to the heat to the outside of the molds to thereby smooth the flowof the resin and prevent the generation of the weld. As a result, it ispossible to enhance the quality of the appearance of the completed upperhalf.

[0138] Also, in the magnetic tape cassette according to the secondaspect of the present invention, the friction sheet used is superior indurability and sliding property. Even if the magnetic tape having thesharp edge is traveled at a high speed, there is almost no sticking ofthe magnetic tape, and the peel of the anti-static agent and the peel ofthe magnetic tape and the friction sheet per se may be suppressed.Accordingly, there is no degradation in performance of the product.

[0139] Also, in the magnetic tape cassette according to the third aspectof the present invention, the detection roller of the auto loader is nothooked at the front lid of the magnetic tape cassette. When the magnetictape cassette is delivered in the auto loader, it is possible to realizethe magnetic tape cassette that is not subjected to a force of openingthe front lid, which may cause the deformation of the magnetic tapecassette.

What is claimed is:
 1. A cassette half molding method for molding one ofupper and lower halves with an injection mold by injection molding, saidupper and lower halves being coupled with each other in a lid alignmentmanner to form a cassette case for accommodating a pair of take-up hubsaround which a magnetic tape is wound, comprising the step of: molding atransparent or semitransparent window portion provided in the upper halfwith said injection mold, wherein a gas vent pin for smoothing a flow ofmolten resin to be introduced in said window portion is provided in saidinfection mold.
 2. A cassette half that is molded with an injection moldby injection molding and that is one of upper and lower halves that arecoupled with each other in a lid alignment manner to form a cassettecase for accommodating a pair of take-up hubs around which a magnetictape is wound, wherein a transparent or semitransparent window portionhaving a take-up hub position regulating rib is provided on said upperhalf, and wherein said upper half is molded by said infection moldprovided with a gas vent pin for smoothing a flow of molten resin at aportion perpendicular to a flow direction of the molten resin of aceiling surface of said take-up hub position regulating rib when thewindow portion is molded by said injection molding.
 3. A magnetic tapecassette comprising: a body formed by coupling an upper half and a lowerhalf with each other in a lid alignment manner; a pair of take-up hubsaround which a magnetic tape is wound and which are rotatablyaccommodated in said cassette body; and a friction sheet interposedeither between said pair of take-up hubs and said upper half, or betweensaid pair of take-up hubs and said lower half, or both, wherein thethickness of said magnetic tape is equal to or less than 7.7 μm, themaximum winding length is equal to or more than 120 m, and the maximumspeed for the tape being wound to the cassette body is 2 m/sec or more,and wherein the friction sheet is made of polyethylene-based resin andhas a surface electric resistance value in the range of 1×10⁷ Ω to1×10¹⁰ Ω.
 4. The magnetic tape cassette according to claim 3, whereinsaid polyethylene-based resin is fluorine-based resin in which a part ofor all of hydrogen atoms contained in molecules constituting a polymercompound are substituted by fluorine atoms.
 5. A magnetic tape cassettewhich is loaded onto a cassette auto delivery apparatus foraccommodating a plurality of magnetic tape cassettes and forsequentially delivering the magnetic tape cassette by a deliverymechanism, comprising: a body case; a first recess formed in a topsurface of said body case for retaining a depression engagement memberhaving a spring and a roller provided in said cassette auto deliveryapparatus for positioning and stopping; a front lid provided swingablyat a front face of said body case for opening and closing an opening fordrawing out a magnetic tape; and a slider disposed in a lower surface ofsaid body case and slidable in backward and forward directions, whereina rib for reducing a resistance when said roller passes through a gapbetween the top surface of the body case and the front lid is providedat a position corresponding to the first recess and on a side of saidfront lid in the top surface of said body case.
 6. The magnetic tapecassette according to claim 5, further comprising a second recesscorresponding to a convex portion of said rib at a position of saidfront lid corresponding to the rib.